1. Field of the Invention
The present invention relates to a power amplifier, and particularly, to a radio frequency (RF) power amplifier which is used in portable wireless devices such as smart phone and mobile phone or application devices such as notebook computer using power battery. More particularly, the present invention relates to a Doherty power amplifier in which a new operation mode for accomplishing high efficiency at a lower output power level is added to operation of a conventional Doherty power amplifier, thereby achieving high efficiency at various output power levels of the power amplifier. The multi-mode Doherty power amplifier to which a second power mode is added may be reduced in size so as to be integrated into a chip.
In a general wireless terminal, an RF power amplifier consumes a considerable portion of power used in the entire terminal system. Therefore, the low efficiency of the RF power amplifier degrades the efficiency of the entire system and wastes a battery, thereby reducing a talk time. For this reason, much research has been conducted to increase the efficiency of the RF power amplifier. When the RF power amplifier has higher efficiency, the battery use may be reduced. Accordingly, the talk time may be extended, or the terminal use time based on battery charging may be increased.
For this reason, the research has been focused on increasing the efficiency of the RF power amplifier. Representative examples of the RF power amplifier which has been recently researched may include a Doherty power amplifier and a multi-mode power amplifier.
The Doherty power amplifier has been first proposed by W. H. Doherty in 1936, and connects a carrier amplifier and a peaking amplifier in parallel to each other using a quarter wave transformer (λ/4 line). A Doherty operation is performed as follows: the Doherty power amplifier changes the amount of current supplied to a load by the peaking amplifier depending on a power level, and controls the load line impedance of the carrier amplifier, thereby increasing efficiency. The multi-mode Doherty power amplifier is configured to operate according to a situation desired by a power amplification stage, and is operated in one mode among various operation modes depending on an output power level.
The Doherty power amplifier configured by adding the multi-power mode characteristics to the conventional Doherty power amplifier may accomplish high efficiency at a lower output power level than in the conventional Doherty power amplifier.
2. Description of the Related Art
The conventional Doherty power amplifier may obtain relatively high power added efficiency at a maximum output point when ideally operating. Furthermore, when the power level is around 6 dB backoff from the maximum output power, the conventional Doherty power amplifier obtain power added efficiency similar to the power added efficiency obtained at the maximum output power. FIG. 1 illustrates power added efficiency (PAE) curves of a conventional ideal Doherty power amplifier and an ideal class-AB power amplifier, and PAE is defined by the following equation:PAE=((Pout−Pin)/Pdc).
The size ratio of the carrier amplifier to the peaking amplifier may be changed to adjust a backoff power point at which relatively high power added efficiency may be obtained.
The multi-mode power amplifier is configured to operate according to a situation desired by the power amplifier stage. Furthermore, the multi-mode power amplifier is operated in one mode of various operation modes depending on an output power level. FIG. 2 is a graph illustrating power added efficiency of the conventional multi-mode power amplifier.
FIG. 3 illustrates a conventional Doherty power amplifier integrated into a chip. FIG. 4 illustrates a specific embodiment of a conventional Doherty power amplifier integrated into a chip. FIG. 5 illustrates a conventional multi-mode power amplifier having a dual-path structure, which uses a switch.
FIG. 6 illustrates a conventional multi-power mode amplifier having a dual-path structure, which does not use a switch. Referring to FIGS. 5 and 6, a power stage required for a second power mode and a power stage required for a first power mode are connected in parallel to each other. That is, FIGS. 5 and 6 illustrate a dual-path structure capable of selecting a path depending on a required power mode. FIG. 7 illustrates a conventional multi-mode power amplifier having a bypass structure, which uses a switch.
FIG. 8 illustrates a conventional multi-mode power amplifier having a bypass structure, which does not use a switch. FIGS. 7 and 8 illustrate a bypass structure capable of bypassing or passing through the maximum power stage without an additional power stage.
The conventional multi-mode power amplifier having a bypass or dual-path structure uses a serial switch to select a path of the power mode. The serial switch must have a size enough to accept the maximum power, in order to transmit large power generated from the power stage to the final output stage. Furthermore, since the switch has a specific loss, a loss caused by a switching operation causes an output power reduction. The reduction may degrade the efficiency of the multi-mode power amplifier.
Korean Patent Laid-Open Publication No. 10-2004-0015107 discloses a bypass multi-mode power amplifier using impedance conversion without using a serial switch. The bypass multi-mode power amplifier uses a combination of one or more passive elements such as capacitors or inductors, in order to implement an impedance converter. Even the passive elements used to implement the impedance converter have a specific loss, even though the loss is smaller than in the serial switch. The loss may cause efficiency reduction.
FIG. 9 is a graph illustrating power added efficiency of the conventional ideal Doherty power amplifier and the conventional multi-mode power amplifier.
The conventional Doherty power amplifier may have relatively high efficiency around a backoff operation point about 6 dB by adjusting load line impedance which is the characteristic of the Doherty power amplifier. However, around a backoff operation point of about 10 dB or more, the conventional Doherty power amplifier has lower power added efficiency than in the conventional multi-mode power amplifier.